Mesh is not smooth, How should I fix this mesh?
The subdivision surface modifier will smooth out the entire mesh by default, which will lose hard edges. You can prevent this by going to Edit mode, Edge selection mode, selecting the "hard" edges (edges that should remain sharp) and choosing Edge -> Edge Crease. When you do this, it will look like nothing is happening, but your mouse is controlling a sliding scale parameter. Click anywhere in the viewport ONCE to complete the operation. You will see a small panel appear in the lower left corner. Open it if it collapsed. Manually set the edge crease in that panel to 1.00. This overrides the value set by your mouse.

Now if you append a subdivision surface modifier to the object, the hard edges will be respected.

If your car's body, tires and rims are all one object, it is likely that not every part requires a subdivision operation. You can separate part of a mesh into its own mesh in Edit mode by selecting the faces you want to detach and typing CTRL + P in the viewport. Choose to detach the selected faces. Even though you are in Edit mode, both meshes will now be selected. You must get out of Edit mode now to limit your selection to one object again.

Tip: if faces on a mesh are difficult to select but no faces on that region are connected to the rest of the mesh, you can type CTRL + + (specifically the "+" symbol on your numpad, not the secondary key) to grow your selection outward from a current selection. If you do not have a numpad on your keyboard (ouch!), you can choose to simulate one in the preferences.